1 files changed, 1 insertions, 0 deletions
diff --git a/stdlib/set.mli b/stdlib/set.mli
index dff78105a..f88480b3c 100644
--- a/stdlib/set.mli
+++ b/stdlib/set.mli
@@ -22,6 +22,7 @@ module type S =
     val equal: t -> t -> bool
     val iter: (elt -> 'a) -> t -> unit
     val fold: (elt -> 'a -> 'a) -> t -> 'a -> 'a
+    val cardinal: t -> int
     val elements: t -> elt list
     val choose: t -> elt
   end